1. Field of the Invention
The present invention relates to a composite substrate used for electronic devices such as display devices, and a method of manufacturing the same.
2. Description of the Related Art
In recent years, to meet the demand for reducing size and weight, a substrate for a liquid crystal panel is being switched from a glass substrate to a plastic substrate. Moreover, to increase a mechanical strength, a plastic substrate including glass fibers or glass beads may be used as a substrate for a liquid crystal panel.
Since the plastic substrate has a high permeability to gases such as oxygen or vapor and a high dimensional change rate due to water absorption, a display defect may occur due to bubbles formed by a permeating gas. If moisture enters a liquid crystal layer, the specific resistance of a liquid display material lowers, which may lead to a display defect. There is also such a problem that a dimensional change caused by moisture absorption during processes causes a poor connection due to a positional shift of transparent electrode circuits on an upper substrate and a lower substrate. In order to solve such problems, at least one surface of the plastic substrate is provided with an inorganic film or a layered film of an inorganic layer and an organic layer (for example, see Japanese Laid-Open Patent Publication No. 2003-251731).
However, an end surface of the plastic substrate is not provided with the inorganic layer or the like. Therefore, in the end surface of the plastic substrate, moisture or gas may enter through an interface between the plastic substrate and the inorganic layer or the like.
Japanese Laid-Open Patent Publication No. 2003-050382 discloses that a cut end surface of a plastic substrate is covered with a coating material by a dipping method or a dispenser method for sealing. However, the inventor found that the method of Japanese Laid-Open Patent Publication No. 2003-050382 has problems as follows. (1) Since the plastic substrate is thin, it is difficult to coat the end surface of the substrate with good adhesiveness. (2) The coating material easily flakes off, which leads to generation of dust. Since the plastic substrate is especially required to have flexibility, the adhesiveness of the coating material to the end surface of the substrate is important. (3) Since an end surface of a plastic substrate including glass fibers or glass beads has the glass fibers or the glass beads which are protruding, the end surface is extremely uneven. Therefore, it is difficult to coat the end surface with the good adhesiveness. Moreover, the difference in thickness of the coating material causes variations in hardening. As a result, flaking of the coating material more easily occurs.
A liquid crystal display device is typically formed by affixing an element substrate provided with elements such as TFTs (Thin Film Transistors) to a color filter (CF) substrate formed with color filters by a sealant and by sealing a liquid crystal material between the substrates. In order to increase the yield of liquid crystal panels, a big mother substrate including two substrates affixed to each other is cut into a plurality of liquid crystal panels. As a method of cutting the mother substrate, a dicing method or a stamping method (such as a Thomson method disclosed in Japanese Laid-Open Patent Publication No. 2003-236794) is generally adopted.
Cutting a plastic substrate including glass fibers or glass beads by the dicing method or the stamping method has problems as follows. (1) Since the glass fibers or the glass beads are bare in the end surface of the substrate, a slit is formed in interfaces between the plastic material and the glass fibers or the like due to a stress gap or the adhesiveness. The slit allows an easy entry of moisture or gas. (2) Since the substrate is mechanically cut, the glass fibers or glass beads fly off or the resin flakes off while the substrate is cut, which leads to generation of dust. (3) Since the glass fibers or the glass beads are bare in the end surface of the substrate and the adhesiveness between the glass fibers or glass beads and the resin in the end surface section is low, injuries or generation of dust may occur during processes after the cutting. (4) The substrate easily cracks due to the lowered adhesiveness or mechanical damages.